High-Throughput Continuous Hydrothermal Synthesis of Nanomaterials

Department of Chemistry, University College London, Christopher Ingold Laboratories, 20 Gordon Street, London, WC1H 0AJ, ... E-mail: [email protected]...
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High-Throughput Continuous Hydrothermal Synthesis of Nanomaterials (Part II): Unveiling the As-Prepared CexZryYzO2−δ Phase Diagram Raul Quesada-Cabrera,† Xiaole Weng,‡ Geoff Hyett,§ Robin J. H. Clark,† Xue Z. Wang,∥ and Jawwad A. Darr*,† †

Department of Chemistry, University College London, Christopher Ingold Laboratories, 20 Gordon Street, London, WC1H 0AJ, United Kingdom ‡ Department of Environmental Engineering, Zhejiang University, Zhejiang Province, Hangzhou, 310027, P. R. China § Department of Chemistry, University of Southampton, Highfield, Southampton, SO17 1BJ, U.K. ∥ Institute of Particle Science and Engineering, School of Process, Environmental and Materials Engineering, University of Leeds, Leeds LS2 9JT, United Kingdom S Supporting Information *

ABSTRACT: High-throughput continuous hydrothermal flow synthesis was used to manufacture 66 unique nanostructured oxide samples in the Ce−Zr−Y−O system. This synthesis approach resulted in a significant increase in throughput compared to that of conventional batch or continuous hydrothermal synthesis methods. The asprepared library samples were placed into a wellplate for both automated highthroughput powder X-ray diffraction and Raman spectroscopy data collection, which allowed comprehensive structural characterization and phase mapping. The data suggested that a continuous cubic-like phase field connects all three Ce−Zr−O, Ce− Y−O, and Y−Zr−O binary systems together with a smooth and steady transition between the structures of neighboring compositions. The continuous hydrothermal process led to as-prepared crystallite sizes in the range of 2−7 nm (as determined by using the Scherrer equation). KEYWORDS: high-throughput, hydrothermal synthesis, phase diagram, Raman spectroscopy, X-ray diffraction

1. INTRODUCTION Nanomaterials in the Ce−Zr−O, Y−Zr−O, and Ce−Y−O binary oxide systems are of interest for a broad range of applications, for example, catalyst supports for elimination of volatile organic compounds (VOCs),1 electrolytes in intermediate-temperature solid-oxide fuel cells for stationary power applications,2 three-way catalysts for cleanup of automotive gases,3 homogeneous efficient fuel source in MOx (mixed U/Pu oxide) nuclear reactors,4 and phosphor materials (e.g. Y2O3:Eu3+).5 Despite the importance of nanoceramics for technological applications, for example, as photocatalysts6,7 or as battery or sensor materials,8 rapid synthesis methods (within a day or less) to generate a large number of nanoceramics in significant quantities (typically 1 g or so) are still underdeveloped. Continuous hydrothermal flow synthesis (CHFS) technology is a rapid way to make nanomaterials and involves mixing a flow of supercritical water with a flow of aqueous metal salt solutions to give rapid nucleation and formation of nanoparticles.6,7,9 The basic continuous hydrothermal technique has been developed independently by several groups. However, phase mapping using this technique remains relatively unexplored considering the vast literature on hydrothermal syntheses.10 This is largely © 2013 American Chemical Society

due to the greater availability of batch hydrothermal processing vessels, compared to the more complex CHFS equipment. Nevertheless, CHFS and related synthesis routes offer important advantages over batch methods, such as rapid nucleation, limited growth,11 the ability to tune or control particle size and shape via changes in temperature/pressure/ reagents,12 and the advantage of producing many materials in a short time in the same apparatus, including the possible access to metastable phases. Some of the authors of this work previously reported the development of a fully automated high-throughput continuous hydrothermal (HiTCH) process which was used for the production of a 66-sample nanoparticle CexZryYzO2−δ library.13 However, the phase diagram of the CexZryYzO2−δ compounds was ascribed after post-treatment of the samples (1000 °C for 1 h in air) to enhance crystallization and promote the resolution of the X-ray diffraction data (during commissioning experiments for the I11 beamline at the Diamond Light Source, Rutherford Appleton Laboratory, Oxford).14 In the current Received: November 3, 2012 Revised: July 23, 2013 Published: August 1, 2013 458

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samples, Raman spectra were also recorded with a UV laser (325 nm) to distinguish the electronic features from vibrational bands.

work, the implementation of high-throughput Raman spectroscopy allowed the investigation of the original as-prepared CexZryYzO2−δ library. These data provided detailed information about the structural changes occurring across the entire CexZryYzO2−δ map. Analysis of the extent of sintering of these nanoparticles upon heating has also been studied by comparing the crystallite sizes of the samples before and after heat treatment, as estimated using the Scherrer equation. To the best of our knowledge, this is the first comprehensive phase map to be reported for the low-temperature synthesis of CexZryYzO2−δ nanoparticles, where some of the binary (such as Ce0.2Y0.8O2−δ and Ce0.1Y0.9O2−δ) and ternary compositions (such as Ce0.1Zr0.3Y0.6O2−δ) were hitherto unknown for asprepared nanoparticles.

3. RESULTS AND DISCUSSION 3.1. Phase Identification. The unequivocal assignment of nanocrystalline phases in the Ce−Zr−Y oxide system could not be established from XRD data alone in the case of the dominant cubic and tetragonal structures in the phase diagram because of broadening of the corresponding reflection peaks. For example, a gradual phase transition from cubic CeO2 to tetragonal ZrO2 should be expected upon incorporation of Zr atoms into the former structure (i.e. moving along the binary Ce−Zr oxide system), under the synthesis conditions employed here. However, the main indication of the substitution of Ce4+ by Zr4+ ions in these patterns is a peak shift related to their different sizes (Figure 1) and the presence of a tetragonal

2. EXPERIMENTAL SECTION 2.1. Synthesis Method. A detailed description of the synthesis of the materials described here is given in the previous work.13 Briefly, a manual high throughput continuous hydrothermal (HiTCH) flow synthesis system was used in which a superheated water feed at 450 °C and 24.1 MPa treated with a flow of appropriate premixed metal salt solutions to form nanoparticles. The precursor solutions were prepared from nitrate precursors (0.3 M Y(NO3)3·6H2O, 0.2 M ZrO(NO3)2· 6H2O, and 0.3 M (NH4)2Ce(NO3)6). The nanoparticle slurries were cooled in flow via a pipe-in-pipe cooler and then the samples exited the process via a back-pressure regulator. Each mixture was fed in sequence with the system being purged with clean water between subsequent samples. Nominal values for metal ion content are used in the description of this work for simplicity instead of the actual compositions (Supporting Information Table S1 and Figure S1) determined by elemental analysis (EDX).13 All slurries synthesized from the HiTCH system were dialyzed and freeze-dried at 12 × 10−3 Torr for 18 h. The powders from the 66 as-prepared samples were placed into an aluminum wellplate library in preparation for analysis. 2.2. High-Throughput Data Collection and Characterization. A data-mining method was introduced to cluster the large volume of XRD and Raman data collected into groups with similar diffraction/vibrational features. A widely used clustering algorithm K-means method was utilized herein, which has significantly accelerated the processing of the data analysis. Details of the clustering have been reported elsewhere.15 High-throughput powder X-ray diffraction (PXRD) was carried out for the as-prepared samples using a Bruker-AXS D8 (GADDS) diffractometer with Cu Kα radiation (λ = 0.15418 nm) at 40 kV and 150 mA over the 2θ range 20−70° with a step size of 0.02° per second. The diffractometer is equipped with an x−y−z translational stage, which can be programmed to select each sample contained in a wellplate for data collection. The crystallite size was estimated from the XRD patterns using the Scherrer equation. The peak broadening because of instrumental contributions was determined using a 25 cm2 alumina flat plate obtained from the National Institute of Standards and Technology (NIST) with peak intensities given by NIST and peak positions by the Inorganic Crystal Structure Database (ICSD). High-throughput Raman studies were conducted using a Renishaw InVia Raman microscope spectrometer equipped with a diode laser operating at 785 nm and position-sensitive detectors. The spectral resolution of the instrument was 1 cm−1. All Raman spectra were recorded at room temperature, averaging 20 scans of 20 s each. For selected

Figure 1. X-ray diffraction patterns from samples in the as-prepared Ce−Zr oxide binary system. All patterns could be refined using a cubic model, despite the presence of tetragonal phases identified using Raman spectroscopy. The peak shift highlighted is because of reduction of the unit cell volume upon substitution of Ce4+ with Zr4+ ions.

structure could only be clearly identified for those samples with very high Zr content (>80%), namely, Ce0.1Zr0.9O2‑δ (S10 in Figure 2) and pure ZrO2 (S11). Nevertheless, the implementation of high-throughput Raman spectroscopy as a complementary technique allowed rapid identification of the predominant phases in the system (Figure 3). The Raman spectra and main band modes of all 66 samples in this study are shown in Supporting Information Figure S1 and Table S2, respectively. The Raman spectrum of the as-prepared cubic CeO2 phase (Fm3m ̅ ) is dominated by a single broad F2g band (symmetric Ce−O stretching vibrations of the fluorite structure) at 465 cm−1 for bulk ceria, but its wavenumber, shape and intensity are very sensitive to particle size and to defects related to oxygen vacancies, strain and phonon confinement.16 In the case of our nanostructured CeO2 sample, the band was observed at 458 cm−1 (Figure 3a). The introduction of relatively small amounts of Zr4+ ions (70−80 at.%) showed evidence of a monoclinic YOOH phase (Supporting Information Figure S2). The pure monoclinic YOOH structure (P21/m) is characterized by nine Raman-active modes (6Ag + 3Bg) (Figure 5c). Among the ternary Ce−Zr−Y oxide samples, the predominant phase was cubic and in a few cases this was in coexistence with a (metastable) tetragonal structure, for example, sample Ce 0.4Zr0.5Y0.1O2−δ (S35) (Figure 4c). However, it was interesting to observe the presence of single-phase pseudocubic structures in nominal compositions such as Ce0.1Zr0.8Y0.1O2−δ (S38) (Figure 4d). When the entire phase diagram is viewed as a whole (Figure 2), there appears to be a continuous solid solution of cubic or pseudocubic phases that pervades the entire phase diagram, connecting each oxide binary system together by means of ternary compositions.

Figure 3. Raman spectra of (a) pure CeO2 (S1), (b) Ce0.6Zr0.4O2 (S5), and (c) Ce0.6Y0.4O2 (S27).

metal−oxygen bond length shortening and a consequent shift of the F2g band from 458 to 464 cm−1 (Figure 3b). In contrast, the substitution of Ce4+ ions by Y3+ ions with similar ionic radii along the Ce−Y oxide binary system did not affect the wavenumber of the main F2g band even for Y contents below 80 at.% (Figure 3c). Nevertheless, the ionic substitution in the CeO2 structure results in the appearance of extra bands in the spectrum. In general, broad bands below 400 cm−1 and in the range 500−600 cm−1 are related to oxygen vacancies and dopant defects. In particular, the presence of a relatively intense broad band at ∼252 cm−1 has been associated with the displacement of oxygen atoms from the ideal fluorite lattice (Figure 3a).17 The additional bands observed at 563 and 612 cm−1 in Figure 3c are due to oxygen vacancies (charge valence in the Ce 4+/Y 3+ substitution) and metal substitution, respectively.18 Above 30 at.% Zr content in the Ce−Zr oxide binary system, the presence of a tetragonal structure is gradually envisaged by changes in the shape and intensity of the F2g band, together with the appearance of a broad band around 145 cm−1 and bands in the range 1200−1800 cm−1 (Supporting Information Figure S2).19 The F2g band is still the dominant feature below 80 at.% Zr content, thus, coexistence between tetragonal and cubic phases can be assumed. However, progressively higher concentrations of Zr atoms in the structure finally result in a pure tetragonal Ce0.1Zr0.9O2−δ (S10) phase 460

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model (or tetragonal model in the case of S10 and S11). On the basis of crystallite size, the as-prepared 66 samples seem to be within the range of 2−7 nm, apparently distributed into three distinct regions (Figure 6a). The compounds in the top half of the phase diagram (high Ce concentration) seem to have crystallite sizes in the range 3.5−4.5 nm, whereas those in the bottom half of the diagram are larger at 5−7 nm. A few samples, particularly those with similar atomic ratios of Ce and Y in the Ce−Y−O binary system, had crystallite sizes in the range ∼2.0−3.5 nm (Figure 7a). Despite these apparent groups, any changes in crystallite size appear to show a smooth transition between neighboring samples, which suggests that the factors controlling their size are subtle and largely down to small changes in composition. A smooth transition in unit cell volume upon changes in composition was also observed in Figure 6. The ionic radii of both Ce4+ and Y3+ ions are very similar (0.97 and 1.01 Å, respectively), whereas Zr4+ ions are substantially smaller (0.87 Å), resulting in unit cell volumes that drop gradually toward the ZrO2 corner of the diagram (Figure 6). Some of the authors involved in the current work previously reported high-quality XRD data of the same library after heat treatment at 1000 °C for 1 h. With the new XRD data herein, it has now been possible to assess the size of the nanoparticles before heat-treatment (Supporting Information Table S3). The difference between crystallite size estimated before and after the heat treatment is represented by the darkness of the spheres in the schematic diagram in Figure 7b. The size of each sphere correlates with the size of the as-prepared samples. A few samples, such as CeO2 (S1), Ce0.9Zr0.1O2−δ (S2), etc., are represented in black or dark gray in the figure; these samples underwent extensive sintering during the heat treatment. Nevertheless, it can be observed that most of the samples did not experience severe sintering after heating at 1000 °C for 1 h (Figure 7b).

Figure 5. Raman spectra of (a) tetragonal/cubic Zr0.4Y0.6O2−δ (S17), (b) cubic Zr0.6Y0.4O2−δ (S15), and (c) monoclinic YOOH (S21).

3.2. Sintering of Nanoparticles. Taking into consideration the limitations of the XRD data, a cubic structure was initially modeled for the extensive and continuous phase field which extended across the entire sample map, except for the clearly tetragonal phases mentioned (S10 and S11) to have a rough estimate of the variation in crystallite size within samples (Figure 6). Some samples containing a high concentration of yttrium, namely S17−S23 and S44−S46, were mixtures of cubic and YOOH phases and could not be refined. Those samples were excluded from the analysis. The crystallite size was roughly estimated using the Scherrer equation from the value obtained for the a parameter in the refinements of the cubic

4. CONCLUSIONS AND OUTLOOK FOR FUTURE RESEARCH An entire as-prepared Ce−Zr−Y ternary oxide nanoparticle phase map has been investigated through the high-throughput complementary techniques of X-ray diffraction and Raman spectroscopy. The nanomaterials were made at relatively low temperatures (certainly none of the nanomaterials referred to herein was exposed to temperatures over 400 °C during synthesis) using a high-throughput CHFS method. Importantly, when compared to our previous report on the samples after heat treatment, this report highlighted that changes in unit cell volume, sintering behavior, initial particle size and phase behavior changed gradually because of the consistency of conditions offered by the HiTCH synthesis method. In CHFS, the particle properties appear to be largely from changes in elemental composition and therefore this dictates solubility and formation in a subtle way across the phase diagram. Further, comparison of crystallite sizes (via analysis of the XRD peak widths and application of the Scherrer equation) before and after heating to 1000 °C for 1 h, revealed that particle growth and sintering were most severe at the corners of the phase diagram and were substantially reduced toward the center of the phase diagram. While the development of high-throughput CHFS (HiTCH) has allowed rapid production of many samples in a relatively short space of time, if any key materials identified from such libraries are to reach commercial interest, it will also be

Figure 6. Estimated unit cell volume from the X-ray diffraction data refined using a cubic structure model for all samples except for pure ZrO2 (S11) and Ce0.1Zr0.9O2−δ (S10) that were clearly tetragonal (according to X-ray data alone). Note in samples containing mixtures of phases, only the major phase was analyzed. The X-ray patterns from the samples with high content of Y could not be refined and are not included in the graph. 461

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Figure 7. (a) Position of the 66 samples in the Ce−Zr−Y oxide system. The color code highlights the different range of crystallite sizes (Scherrer) of the as-prepared samples. The samples with high Y content (gray spheres) were varying mixtures of apparently cubic and YOOH phases and have been excluded from the analyses. (b) Schematic diagram illustrating the relative crystallite size of the 66 samples before and after heat treatment (1000 °C for 1 h, ref 13). The relative crystallite size estimated for the as-prepared samples correlates with the size of the spheres shown, whereas their gray scale indicates the degree of sintering after the heat-treatment (i.e., the difference in crystallite size). The degree of sintering is also highlighted by the two red dashed circles that illustrate the size of two stereotypical samples (S2 and S7) after the heat treatment. A full explanation is described in Supporting Information. The negligible sintering of the nanoparticles in sample S7 contrasts with the drastic change in crystallite size of sample S2 upon heat-treatment.

Foundation of China (LQ12E08011) and The National Natural Science Fundation of China (Grant No. 51208458).

important to scale up CHFS technology. Toward this end, the authors have recently developed a pilot plant scale CHFS, which uses a new mixer design and is capable of producing >1 kg per hour of nanomaterials (×800 scale-up) with similar characteristics to those obtained in the lab-scale reactors.25 Thus, the benefit of high throughput synthesis and characterization and then rapid scale-up production for nanoceramics are within grasp of researchers.

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ASSOCIATED CONTENT

S Supporting Information *

Raman spectra of the entire Ce−Zr−Y−O phase diagram and unit cell parameters and crystallite sizes for the 66 samples before and after heat treatment. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +44 (0)20 7679 7463. Tel: +44 (0)20 7679 4345. E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Sonal Brown for assistance in Raman data collection. J.A.D. and R.Q.C. were funded by EPSRC under “Continuous Hydrothermal Synthesis of Nanomaterials: From Laboratory to Pilot Plant” [EP/E040551/1]. X.W. was financially supported by Zhejiang Provincial Natural Science 462

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dx.doi.org/10.1021/co3001346 | ACS Comb. Sci. 2013, 15, 458−463